The Detection of Cell-Bound Antibody on

nism by which human red cells, in a variety of clinical settings, become coated in vivo with autologous com-plement components in the absence of anti-red cell autoantibodies demonstrable by standard methods. By means of a newly developed complement-fixing antibody consumption test, previously undetectable red cell-bound YG globulin could be detected and quantified. By this technique, the complement-coated red cells of 13 of 16 patients were shown to carry abnormally high numbers of aG molecules per cell, which were never-theless below the level for detection by the direct anti-globulin test. Eluates were made from the red cells of seven of these patients and each eluate, when sufficiently concentrated, was capable of sensitizing normal human red cells (with vG antibodies) to give a positive indi-rect antiglobulin test with anti-'vG serum. In the pres-ence of fresh normal serum, six of the eluates so tested were capable of fixing complement to normal human red cells. The antibodies in the red cell eluates did not exhibit Rh specificity and did not react with nonprimate red cells. When studied by sucrose gradient ultracentri-fugation, the vG antibodies to human red cells in these eluates sedimented in the 7S region. It is concluded that in many patients in whom direct antiglobulin tests re-veal only cell-bound complement, the complement fixa-tion is mediated in vivo by small quantities of "warm-reacting " erythrocyte autoantibodies of the aG class